1. Field of the Invention
The present invention relates to an oven controlled crystal oscillator (OCXO: Oven Controlled Crystal Oscillator). More specifically, the present invention relates to an oven controlled crystal oscillator in which, in a case where a metal lead is soldered to a substrate, cracks are hard to occur in the solder and its reliability can be secured, and a production method of the oven controlled crystal oscillator.
2. Description of the Related Art
[Prior Art]
In order to increase frequency stability, an oven controlled crystal oscillator attempts to stabilize frequency by controlling, in a thermostatic oven, the temperature of components which are largely affected by temperatures in a wide temperature range.
The temperature control in the oven controlled crystal oscillator is generally carried out such that a control circuit that controls a thermostatic oven carries out temperature control with the use of a differential direct-current amplifier by a resistance bridge using a thermistor. When on/off of a power source of the thermostatic oven is repeated, a heat cycle is burdened on a using component/material, which causes a problem in reliability.
[Related Art]
Note that as related conventional techniques, there are the followings: Japanese Patent Application Laid-Open No. 2010-080945, “Method for Soldering Through-hole Mount Component, Soldering Structure of Through-hole Mount Component, and Electronic Circuit Substrate” (Sumitomo Electric System Solutions Co., Ltd.) [Patent Document 1]; Japanese Patent Application Laid-Open No. 2005-303612, “Crystal Resonator” (NIHON DEMPA KOGYO CO., LTD.) [Patent Document 2]; and Japanese Patent Application Laid-Open No. 2010-093536, “Constant-Temperature Crystal Oscillator” (NIHON DEMPA KOGYO CO., LTD.) [Patent Document 3].
Patent Document 1 discloses that: in an electronic circuit substrate, a cream solder is applied to a through-hole land of a component side and heated to be solidified, thereby forming a pre-tinning solder; then, a lead is inserted into the through hole of a substrate on which the pre-tinning solder is formed; and a solder is supplied to the through hole so as to be integrated with the pre-tinning solder.
Patent Document 2 discloses that, in a crystal resonator using a lead-free solder, in order to prevent a metal pool, a lead-free solder layer of a lead wire assumed for earth connecting is formed with some distance from a bottom surface of a metal base, which distance is larger than the distance from the bottom surface of the metal base to a lead-free solder layer of a pair of lead wires, and a recess is provided on the bottom surface of the metal base to which the lead wire for earth connecting is led.
Patent Document 3 discloses that, in a constant-temperature crystal oscillator, a circuit substrate is held by lead wires (first lead wires) made airtight by a glass of a oscillator base (a metal base), and ends of an extraction electrode which are extended from a drive electrode of a crystal blank is held by a supporter in which a crystal resonator is connected to a pair of lead wires (second lead wires) of the metal base.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2010-080945
[Patent Document 2] Japanese Patent Application Laid-Open No. 2005-303612
[Patent Document 3] Japanese Patent Application Laid-Open No. 2010-093536
However, such conventional crystal oscillators have the following problem. That is, in a case where a glass epoxy resin is used for a circuit substrate, when a lead terminal for connecting the circuit substrate to a base, a pin for connecting it to another circuit substrate, or a lead-terminal equipped crystal resonator is soldered, a distortion centers on an applied solder in an operating environment in which a heat cycle occurs, because of a difference in linear expansion coefficient between the substrate made from a glass epoxy resin material and the metal terminal. This results in an occurrence of cracks in the solder.
Here, as to the linear expansion coefficient of the glass epoxy resin material, for a common CEM (Composite Epoxy Material) −3, the coefficient is 25 ppm/° C. in a longitudinal direction, 28 ppm/° C. in a lateral direction, and 65 ppm/° C. in a thickness direction, and for a common FR (Flame Retardant) −4, the coefficient is 13 ppm/° C. in a longitudinal direction, 16 ppm/° C. in a lateral direction, and 60 ppm/° C. in a thickness direction.
Particularly, the oven controlled crystal oscillator (OCXO) has such a problem that in an operating environment in which on/off of a power supply is repeated, that is, an operating environment in which a heat cycle occurs, a temperature change from an ambient temperature to a thermostatic-oven-controlling temperature (e.g., 85° C.) is caused. In view of this, when a metal lead is soldered to a substrate made from a glass epoxy resin material, cracks occur in the solder, thereby resulting in that reliability cannot be improved.
Further, Patent Documents 1 and 2 do not take into consideration that a metal lead can be stably attached to a substrate and that a decrease in reliability can be prevented even if any crack occurs in a solder.